Passivity breakdown mechanism localized corrosion

Analysis of passivation transients on an initially active surface either by applying a steep potential jump into the passive range or by creating fresh surfaces at constant applied potential by nonelectrochemical depassivation (chemical passivity breakdown mechanical scratching, ultrasonic waves, etc. radiative laser beam impact [112,113]). These techniques have proved to be of outstanding importance for the investigation of the mechanism of localized corrosion associated with passivity breakdown [114,115]. 

The following mechanisms in corrosion behavior have been affected by implantation and have been reviewed (119) (/) expansion of the passive range of potential, (2) enhancement of resistance to localized breakdown of passive film, (J) formation of amorphous surface alloy to eliminate grain boundaries and stabilize an amorphous passive film, (4) shift open circuit (corrosion) potential into passive range of potential, (5) reduce/eliminate attack at second-phase particles, and (6) inhibit cathodic kinetics. 

H. Bohni, Breakdown of passivity and localized corrosion processes, Langmuir 3 (1987) 924—930. F. Hunkeler, G.S. Frankel, H. Bohni, On the mechanism of localized corrosion. Corrosion 43 (1987) 189-191. 

Localized corrosion of metals and alloys occurs in aggressive media (e.g., containing chloride) as a consequence of the passivity breakdown, with major impact in practical applications and on the economy. This form of corrosion is particularly insidious since a component, otherwise well protected by a well-adherent, ultrathin oxide or oxyhydroxide barrier layer (i.e., the passive film), can be perforated locally in a short time with no appreciable forewarning. Extensive studies have been conducted over the last five decades to understand localized corrosion by pitting [1-10], but the detailed mechanisms accounting for the local occurrence of passivity breakdown remain to be elucidated and combined with kinetics laws to allow reliable prediction. 

Under certain special environmental conditions, the passive films, which were described earlier in this Chapter, are susceptible to localized breakdown. Passivity breakdown may result in accelerated local dissolution (localized corrosion) of the metal or alloy. There are two (related) major forms of localized corrosion following passivity breakdown localized corrosion initiated on an open surface is called pitting corrosion, and localized corrosion initiated at an occluded site is called crevice corrosion. In the presence of mechanical stress, localized dissolution may promote the initiation of cracks. 

Specific anion dependence is expected to occur in the passive andtianspassive domains, and dissolution in the active range can be made to deviate from the hydroxo-ligand mechanism [87] only by anions able to replace OH, essentially SH" [88] and the halide ions. In the case of iron, due to the well-known passivity breakdown and subsequent localized corrosion by halide ions and particularly Cl , chloride effects have been investigated extensively. Complexing anions such as acetate have also been considered to a lesser extent. 

The role of sulfide inclusions in corrosion has been recognized in early woiks. The fact that sulfur-containing species are detrimental to the lesistanee of metals and alloys to localized corrosion has been established for a long time but the meehanisms have remained unclear until recently. In the area of passivity breakdown, where substantial research effort has been expended for several years, the effects of chloride ions have been investigated much more than the effects of sulfur. The aim of this chapter is to review the fundamental aspects of the mechanisms of S-induced corrosion, with special emphasis on the role played by adsorbed (or chemisorbed) sulfur. 

In the case of the nickel alloys, the stability of the passive layer is a problem. The alloys depend on the oxide films or the passive layers for corrosion resistance and are susceptible to crevice corrosion. The conventional mechanism for crevice corrosion assumes that the sole cause for the localized attack is related to compositional aspects such as the acidification or the migration of the aggressive ions into the crevice solution [146]. These solution composition changes can cause the breakdown of the passive film and promote the acceleration and the autocatalysis of the crevice corrosion. In some cases, the classic theory does not explain the crevice corrosion where no acidification or chloride ion build up occurs [147]. 

For pit nucleation, defects within the metal surface also have to be considered. Inclusions like MnS may prevent the formation of a continuous, protecting passive film locally. They are preferential sites for a breakdown of passivity. Even in this situation, the special chemical properties of aggressive anions to start corrosion pits and to cause their continuous growth have to be included in the proposed mechanisms. These properties should be explained along with the ability of halides to form stable and fast-dissolving complexes of the cations of the metals under study with the related breakdown of their passive layers. 

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